表示推测的情态动词
情态动词must, can / could, may / might均可表示推测，它们可以对过去、现在或将来的情况作出语气强弱不同的推测。在运用情态动词表示推测时，我们应该着重把握以下两点：
1. 把握推测语气的特点，选择恰当的情态动词。
◆表示肯定的推测时，各情态动词语气从弱到强依次为might → may → could → can → must;表示否定的推测时，can’t / couldn’t语气较强，意为“不可能”;may not / might not语气较弱，意为“可能不”。
注意：must的否定形式mustn’t不表示否定推测，而是表示“不许可”或“不应该”。
◆情态动词表示推测时，在用法上有一些限制：
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must只用于肯定句中，意为“一定，准是”。如：They have been working hard all day. They must be tired.
can多用于否定句或疑问句中。could可用于各种句式。如：
Can the story be true?
Simon can’t be at home, for I saw him going shop- ping just now.
You mustn’t smoke when you are walking around in the wood. You could start a fire.
may / might用于肯定句和否定句中。如：
It may or may not rain. I’m not sure.
Peter might come with us tonight, but he isn’t very sure yet.
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2. 确定被推测时间，正确判断动词时态。
◆对现在或将来的情况进行推测时，用“情态动词+ do”。如：
Mr. Reed looks pale. He may be ill.
If you don’t have a guide, you could lose your way.
◆对此时此刻正在进行的情况进行推测时，用“情态动词+ be +动词-ing形式”。如：
At this moment, our teacher must be correcting our exam papers.
He could be doing his homework in his room now.
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He may be waiting for you now.
◆对过去的情况进行推测时，用“情态动词+ have +过去分词”。如：
The road is wet. It must have rained last night.
Mars couldn’t have watched TV yesterday for he knew they would have an exam.
He didn’t come to school yesterday. He might have been ill.
【拓展】在特定的语境中，“might / could + have +过去分词”不是对一件事情是否发生进行推测，而是表示本来可能发生但没有发生，或者本来可能完成却没有完成的动作，有时具有一定的感情色彩。
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①might have done本可以做(却没有)，有时表示很委婉的责备。如：You might have phoned me, though you were busy then.
②could have done本可能做(却没有)。如：
He could have caught the early bus, but he was delayed by helping a stranger.